1. Field of the Invention
The invention relates to Schottky diode structures, and more particularly to Schottky diode structures characterized by high cutoff frequencies, wide bandwidth and low parasitic reactance.
2. Description of the Related Art
The Schottky barrier mixer diode is widely used in ultra-broad bandwidth (on the order of 10 GHz or greater) millimeter-wave receivers beyond 100 GHz. Such mixer-down converters offer the combination of low cost, wide band-width, large dynamic range, good sensitivity and room temperature operation virtually unparalleled by alternative technology. However, the RF performance of the mixer circuit depends heavily on the electrical characteristics of the Schottky barrier diodes employed.
In a millimeter-wave balanced mixer, the diodes are preselected in matched pairs to minimize undesirable noise conversion from the local oscillator to the IF output. A mixer diode can be regarded as the combination of a nonlinear, voltage dependent series resistance and capacitance in parallel with a stray capacitance. The commonly used figure of merit of a Schottky mixer diode f.sub.co (cut-off frequency) is defined as EQU f.sub.co =(1/2 R.sub.s C.sub.o) (1)
where R.sub.s is the series resistance of the Schottky diodes, C.sub.o is the total capacitance across the diode at zero volt applied bias, representing the sum of C.sub.jo and C.sub.stray, where C.sub.jo is the depletion capacitance of the unbiased Schottky junction, and C.sub.stray is the parasitic capacitance between the electrodes of the diodes.
Normally, an external shunt inductance L is employed in a mixer circuit so that the capacitance and the inductance are in resonance at the midband frequency where the mixer exhibits minimum conversion loss. As the operating frequency moves away from the midband frequency, the RF performance of the mixer starts to deteriorate due to increasing impedance mismatch. It may be concluded from impedance matching considerations that the operating bandwidth of a mixer is inversely proportional to the product of the parasitic capacitance and the midband frequency.
In addition to the electrical performance parameters, physical integrity of a beam lead mixer diode is of high importance in actual millimeter-wave circuit applications. Circuit fabrication yield, mixer manufacturing cost and component survivability under extreme temperature cycling, vibration, and acceleration conditions are strongly dependent on the mechanical stability of the device measured in terms of bondability and beam lead pull strength. A number of device configurations are commonly used for beam lead mixer diodes.
One conventional configuration is the mesa-type beam lead GaAs Schottky mixer diode, wherein a low permittivity dielectric (dielectric constant lower than 12) beam lead support serves to minimize the stray capacitance between the beam lead and the mesa, resulting in high device cut-off frequency and low parasitic capacitance. However, this type of device is extremely fragile (the typical beam lead pull strength is less than 5 grams). Device characterization and mounting are found to be very difficult to accomplish even for skilled workers.
Another conventional configuration is the modified mesa-type beam lead diode, which employ a dielectric bridge to insulate the Schottky electrode from the N.sup.+ doped mesa. This diode has the same advantages of the regular mesa-type diode in its high device cut-off frequency and low parasitic capacitance. In addition, the beam leads which are anchored on the substrate are far more rugged than those attached to the dielectric (glass) support. Unfortunately, the thickness uniformity of the dielectric layer at the vertical walls of the mesa is not subject to tight processing control, resulting in wide variations of stray capacitance from one diode to another. Such device parameter non-uniformity is not acceptable for balanced mixer configurations. Moreover, the etched mesa, which is typically 3 microns in thickness, poses severe compatibility problems with the fine line photolithographic technique employed to define the typical 1 micron.times.10 micron Schottky junction. Consequently, the processing yield of this type of diode is far from satisfactory. This device is also fragile, having a typical beam lead pull strength of less than 5 grams.
A truly planar mixer diode fabricated using a proton bombardment technique to provide isolation between the electrodes of Gallium Arsenide (GaAs) mixer diode devices is presently being marketed by the assignee of the present application. This configuration eliminates the dielectric beam lead support entirely, relying on a carefully controlled proton bombardment to destroy the crystalline structure over selected regions of the N.sup.+ surface region to convert these selected areas into non-conductive regions. This device configuration offers excellent performance uniformity, manufacturing yield, and mechanical integrity. However, the bombarded N.sup.+ regions are of relatively high permittivity, resulting in an increased parasitic capacitance.